Granule exocytosis is an essential platelet function, contributing not only to hemostasis and thrombosis, but also to inflammation, wound healing, malignancy, and angiogenesis. Previous efforts to understand the molecular mechanisms of granule exocytosis have identified the SNARE machinery and chaperone proteins required for membrane fusion. Studies described in this proposal will evaluate the premise that this machinery serves a role in sorting specific cargos to distinct subpopulations of -granules and enabling differential release of granule contents upon platelet activation. We will study granule segregation and secretion in mice lacking VAMP-3, VAMP-5, VAMP-7, and VAMP-8. Cytoskeleton dynamics that occur during platelet spreading can influence SNARE interactions and modify granule exocytosis. We will evaluate granule exocytosis during platelet spreading. These studies will also assess the role of exocytosis in providing membrane for growing actin structures at the periphery of adherent platelets and for directed spreading and movement on micropatterned matrices of adhesion proteins. Use of high-speed intravital microscopy will be used to monitor release of granules from individual platelets during thrombus formation in vivo. These approaches will allow us to evaluate the hypothesis that the mechanism of granule secretion is altered in adherent platelets compared to suspension platelets. We will also evaluate the role of dynamins in controlling fusion pore dynamics, demonstrating a level of control of platelet exocytosis that occurs following SNARE-mediated membrane fusion. These studies will use single-cell amperometry to monitor the release of individual granules from single platelets. The role of dynamins in controlling platelet exocytosis during thrombus formation in vivo will evaluate the hypothesis that the platelet fusion pore can be regulated to control thrombus formation. These experiments will address poorly understood aspects of platelet granule exocytosis and provide a foundation for targeting select platelet membrane fusion events during thrombosis.